A PET scanner includes a ring of detectors for detecting gamma-rays. For each pair of events detected by diametrically opposed detectors, these exists a probability that those events arose from a pair of gamma-rays received directly from within a patient. However, there also exists a probability that at least one of the received gamma-rays was scattered on its way to the detector, or that the gamma-rays did not even arise from the same annihilation event.
In rendering an image, the latter two types of event pairs contribute to noise. It is therefore desirable to suppress such noise when generating the image.